Formation and growth of atmospheric nanoparticles in the eastern Mediterranean: results from long-term measurements and process simulations

Formation and growth of atmospheric nanoparticles in the eastern Mediterranean

Formation and growth of atmospheric nanoparticles in the eastern Mediterranean: results from long-term measurements and process simulationsFormation and growth of atmospheric nanoparticles in the eastern MediterraneanNikos Kalivitis et al.

Atmospheric new particle formation (NPF) is a common phenomenon all over
the world. In this study we present the longest time series of NPF records in
the eastern Mediterranean region by analyzing 10 years of aerosol number size
distribution data obtained with a mobility particle sizer. The measurements
were performed at the Finokalia environmental research station on Crete,
Greece, during the period June 2008–June 2018. We found that NPF took place
on 27 % of the available days, undefined days were 23 % and non-event
days 50 %. NPF is more frequent in April and May probably due to the
terrestrial biogenic activity and is less frequent in August. Throughout the
period under study, nucleation was observed also during the night. Nucleation
mode particles had the highest concentration in winter and early spring,
mainly because of the minimum sinks, and their average contribution to the
total particle number concentration was 8 %. Nucleation mode particle
concentrations were low outside periods of active NPF and growth, so there
are hardly any other local sources of sub-25 nm particles. Additional
atmospheric ion size distribution data simultaneously collected for more than
2 years were also analyzed. Classification of NPF events based on ion
spectrometer measurements differed from the corresponding classification
based on a mobility spectrometer, possibly indicating a different
representation of local and regional NPF events between these two measurement
data sets. We used the MALTE-Box model
for simulating a case study of NPF in the eastern Mediterranean region.
Monoterpenes contributing to NPF can explain a large fraction of the observed
NPF events according to our model simulations. However the adjusted
parameterization resulting from our sensitivity tests was significantly
different from the initial one that had been determined for the boreal
environment.

Most of the atmospheric aerosol particles, and a substantial fraction of
particles able to act as cloud condensation nuclei (CCN), have been
estimated to originate from new particle formation (NPF) taking place in the
atmosphere (Spracklen et al., 2006; Kerminen et al., 2012; Gordon et al.,
2017). The exact mechanisms driving atmospheric NPF and subsequent particle
growth processes are still not fully understood nor are the roles of
different vapors and ions in these processes (Kulmala et al., 2014;
Lehtipalo et al., 2016; Tröstl et al., 2016). In order to understand how
aerosol particles affect regional and global climate and air quality, it is
necessary to quantify the factors that determine the occurrence of NPF and
characterize the parameters that describe the strength of NPF, such as the
new particle formation and growth rates, in various environments.

While NPF has been reported to take place worldwide (Kulmala et al., 2004a;
Wang et al., 2017), observational studies on this subject are scarce in rural
subtropical environments. It has been shown that the processes responsible
for particle formation and growth differ substantially across the European
continent (Dall'Osto et al., 2018).

Several studies have investigated NPF in the eastern Mediterranean and found it
to be a frequent phenomenon. Lazaridis et al. (2006) first reported NPF in the area and correlated these events with polluted air masses. Petäjä
et al. (2007) presented NPF in the Athens metropolitan area and showed that
under the influence of urban pollution, condensing species leading to growth
of the new particles are far more hygroscopic than under cleaner conditions.
NPF events have also been reported to be frequent in the urban environment
of Thessaloniki (Siakavaras et al., 2016). Kalivitis et al. (2008) showed
that precursors and nucleation mode particles experience strong scavenging
on Crete during summer. Pikridas at al. (2012) suggested that
nucleation events occurred only when accumulation mode particles were
neutral, being consistent with the hypothesis that a lack of NH3,
during periods when particles are acidic, may limit nucleation in
sulfate-rich environments such as the eastern Mediterranean. Additionally,
based on ion observations, Pikridas et al. (2012) showed that NPF is more
frequent in winter. By using the same data set from the eastern Mediterranean,
Kalivitis et al. (2012) reported nighttime enhancements in ion
concentrations with a plausible association with NPF, being among the very
few locations where such observations have been made. Manninen et al. (2010)
presented an analysis of a full year of observations of NPF with atmospheric
ion spectrometers at various locations across Europe during the EUCAARI
project and showed that NPF is less frequent at the eastern Mediterranean
site than at other, mostly continental, European sites. On the other hand,
Berland et al. (2017) showed that similar patterns are being observed
throughout the Mediterranean when comparing observations from the island of
Crete to a western Mediterranean site in terms of the frequency of
occurrence, seasonality, and particle formation and growth rates. Kalivitis
et al. (2015), for the first time, studied the NPF–CCN link using observations
of particle number size distributions, CCN and high-resolution aerosol
chemical composition for the eastern Mediterranean atmosphere. From the
hygroscopicity of the particles in different size fractions, it was
concluded that smaller particles during active NPF periods tend to be less
hygroscopic (and richer in organics) than larger ones. Finally, Kalkavouras
et al. (2017) reported that NPF may result in higher CCN number
concentrations, but the effect on cloud droplet number is limited by the
prevailing meteorology.

In this work, we present results from the analysis of 10 years of aerosol
particles number size distributions and more than 2 years of atmospheric
ion size distributions, representing one of the longest published NPF data
set in the Mediterranean atmosphere. The main questions we wanted to address
were as follows: (1) How often does NPF take place in the eastern Mediterranean, what are
the characteristics of this phenomenon and to what extent has it changed
over the period under study? (2) Are there features in NPF observed at the
study area that are not common in other locations? (3) How well can
numerical models, used in different environmental conditions, represent NPF
in this subtropical environment?

2.1 Measurements

Measurements presented in this work were carried out at the atmospheric
observation station of the University of Crete at Finokalia, Crete, Greece
(35∘20′ N, 25∘40′ E; 250 m a.s.l.) over 10 years, between June 2008 and June
2018. The Finokalia station (http://finokalia.chemistry.uoc.gr/, last access: 20 February 2019) is a
European supersite for aerosol research, part of the ACTRIS (Aerosols,
Clouds, and Trace gases Research Infrastructure) Network. The station is
located at the top of a hill over the coastline, in the northeast part of
the island of Crete (Mihalopoulos et al., 1997). The station is
representative of the marine background conditions of the eastern Mediterranean
(Lelieveld et al., 2002), with negligible influence from local anthropogenic
sources. The nearest major urban center in the area is Heraklion with
approximately 200 000 inhabitants, located about 50 km to the west of the
station.

In order to monitor the NPF events, a TROPOS-type custom-built scanning
mobility particle sizer (SMPS), similar to the
IFT-SMPS in Wiedensohler et al. (2012), was used at Finokalia. Particle
number size distributions were measured in the diameter range of 9–848 nm
every 5 min. The system was a closed loop, with a 5 : 1 ratio between the
aerosol and sheath flow, and it consisted of a Kr-85 aerosol neutralizer
(TSI 3077), a Hauke medium differential mobility analyzer (DMA) and a
TSI-3772 condensation particle counter (CPC). The sampling was done through a
PM10 sampling head, and the sample humidity was regulated below the
relative humidity of 40 % with the use of
Nafion® dryers in both the aerosol and sheath
flow. The measured number size distributions were corrected for particle
losses by diffusion on the various parts of the SMPS according to the
methodology described in Wiedensohler et al. (2012). Three different types of
calibration were performed for the SMPS, DMA voltage supply calibration,
aerosol and sheath flows calibrations and size calibrations. These
measurements have been performed at Finokalia on a continuous basis since
2008. The instrument used at Finokalia was audited on-site with good results
in the framework of the EUSAAR (European Supersites for Atmospheric Aerosol
Research) project
(http://www.wmo-gaw-wcc-aerosol-physics.org/audits.html, last access: 20 February 2019) and has successfully passed laboratory
intercomparison workshops twice (2013 and 2016, reports available at
http://www.wmo-gaw-wcc-aerosol-physics.org/instrumental-workshops.html, last access: 20 February 2019) in the framework of the ACTRIS project. The
instrument has been operated following the recommendations described in
Wiedensohler et al. (2012). Additional information for newly formed particles
was obtained with the use of an air ion spectrometer (AIS-AIREL Ltd.,
Institute of Environmental Physics, University of Tartu, Estonia; Mirme et
al., 2007). AIS is a cluster ion air spectrometer used to simultaneously
measure electrical mobility distribution of positive and negative air ions
(mobilities in the range of 2.4 to 1.3×10-3 cm2 V−1 s−1). The mobility distributions were then
transformed to size distributions in the size range 0.8–42 nm. The number
counting threshold was approximately 10 cm−3 and the uncertainties of
the AIS measurements were ∼10 % for negative and positive ion
concentrations and ∼0.5 nm for size (Manninen et al., 2010). The
diameter of the AIS inlet tube was 35 mm and the sample flow rate was
60 L m−1. The time step of the measurements was 5 min.

These measurements have been used to identify NPFs for the whole period and
provide a historical perspective for the frequency and the characteristics
of NPF phenomena in the eastern Mediterranean. Calculations for formation
rates of new particles (J), growth rates (GRs) in various size ranges and
condensation sink (CS) were made according to Kulmala et al. (2012). Formation
rates of particles with diameter Dp (in this study Dp=9 nm) were
calculated as

(1)JDp=ΔNDpΔt+CoagS⋅NDp+GRΔDp⋅NDp+Slosses.

ΔNDp is the increase in nucleation mode particles'
number concentration (Dp<25 nm), CoagS is the coagulation of 9 nm
particles on larger particles and GR is the growth rate in the size range 9–25 nm. Slosses takes into account additional losses and was ignored in
this study. GR was calculated using the mode-fitting method (Dal Maso et al.,
2005). The aerosol size distributions were fitted with lognormal
distributions, and the nucleation mode geometric mean diameter was plotted as
a function of time. GR was calculated as the slope of the linear fit so that

(2)GR=dDpdt.

CS is the condensation sink caused by the preexisting aerosol population and
was calculated using the properties of sulfuric acid as condensing vapor.

All important meteorological parameters were monitored every 5 min
using an automated meteorological station, including the temperature, wind
velocity and direction, relative humidity, solar irradiance, and
precipitation. Ozone concentrations were measured with a TEI 49C instrument
and nitrogen oxides with a TEI 42CTL, both commercially available, with a
time step of 5 min.

2.2 NPF simulations with the MALTE-Box model

The simulations of NPF events in the eastern Mediterranean atmosphere were
here performed using the MALTE-Box model of the University of Helsinki. This
0-D model able to simulate aerosol dynamics and chemical processes has
successfully reproduced observations of aerosol formation and growth in the
boreal environment (Boy et al., 2006) as well as in highly polluted areas
(Huang et al., 2016). For the present study, chemical reactions relevant to
the production of condensing species from the Master Chemical Mechanism
(MCM) were incorporated in the MALTE-Box chemical mechanism, as described in
Boy et al. (2013). These include the full MCM degradation scheme of the
following volatitle organic compounds (described in more detail in
Tzitzikalaki et al., 2017): C1-C4 alkanes, C2-C3
alkenes, acetylene, isoprene, α- and β-pinene, aromatics,
methanol, dimethyl sulfide, formaldehyde, formic and acetic acids,
acetaldehyde, glycoaldehyde, glyoxal, methylglyoxal, acetone,
hydroxyacetone, butanone, and marine amines. The Kinetic PreProcessor (KPP)
was used to produce the Fortran code for the calculations of the
concentrations of each individual compound (Damian et al., 2002), except for
those species whose concentrations were manually input from large-scale
model simulations.

The major aerosol dynamical processes for clear-sky atmosphere were simulated
by the size-segregated aerosol model UHMA (University Helsinki Multicomponent
Aerosol Model; Korhonen et al., 2004)
included in the MALTE-Box model. Measured aerosol
number size distributions were used to initialize UHMA daily, which simulates
NPF, coagulation, growth and dry deposition of particles. UHMA simulated new
cluster formation using the activation nucleation parameterization, so that
the nucleation rate has a linear relationship with sulfuric acid
concentration, depending on the nucleation coefficient Kact.

Apart from sulfuric acid, about 20 extremely low-volatility organic
compounds (ELVOCs) and 7 selected semi-volatile organic compounds (SVOCs)
were treated as condensing vapors, following the simplified chemical
mechanism presented in Huang et al. (2016). All condensing compounds were
treated either as sulfuric acid or organic compounds, and the condensation of
organic vapors was determined by the nano-Kohler theory (Kulmala et al.,
2004b).

The observations at Finokalia station were used as input to the MALTE-Box
model, and when such observations were not available, the results from
numerical simulations with the global three-dimensional chemistry transport
model (CTM) TM4-ECPL (Daskalakis et al., 2015, 2016; Myriokefalitakis et al.,
2010, 2016) for Finokalia were used. Observational data include temperature,
relative humidity, total radiation (meteorological input), ozone
(O3) and nitrogen oxides (NOx) concentrations as
well as aerosol number size distributions. The aerosol number size
distribution measured by the SMPS was used to calculate the condensation sink
for H2SO4 vapors. Due to the lack of detailed measurements of
volatile organic compounds (VOCs) at
Finokalia, as a first approximation, biogenic and anthropogenic
concentrations of all the abovementioned VOCs resolved every 3 h were taken
from the TM4-ECPL model.

The global TM4-ECPL model was run driven for this study by ECMWF (European
Centre for Medium – Range Weather Forecasts) Interim re-analysis project
(ERA – Interim) meteorology (Dee et al., 2011) of
the year 2012 at a horizontal resolution of 3∘ in longitude ×2∘
in latitude with 34 vertical layers up to 0.1 hPa. The model used
year-specific meteorology and emissions of trace gases and aerosols. For
this study, that of the year 2012 was used, except for soil NOx and oceanic
CO and VOC emissions, which were taken from the POET inventory database for the
year 2000 (Granier et al., 2005). TM4-ECPL simulations for this work were
performed with a model time step of 30 min, and the simulated VOC
concentrations every 3 h were used as input to the MALTE box model; SO2 surface levels at Finokalia were taken from the Monitoring Atmospheric
Composition and Climate (MACC) data assimilation system (Inness et al.,
2013).

For the calculations of the photodissociation rate coefficient by the
MALTE-Box model, the solar actinic flux (AF) is needed. Unfortunately, AF was
not measured at Finokalia in 2012; therefore, AF levels were calculated by the
Tropospheric Ultraviolet and visible Radiation Model (TUV; Madronich, 1993)
version v.5 for cloud-free conditions. The ability of TUV to calculate the AF
at Finokalia was investigated by comparing observations of photodissociation
rates of O3 (JO1D) and NO2
(JNO2) and model calculations. The measurements of these
photodissociation rates were performed by filter radiometers (Meteorologie
Consult, Germany). The JO1D was measured at wavelengths <325 nm, while for JNO2 wavelengths <420 nm were
used.

A series of sensitivity tests of AF to different input parameters was also
performed to optimize the calculations. The model uses extraterrestrial solar
spectral irradiance (200–1000 nm by 0.01 nm steps) and computes its
propagation through the atmosphere taking into account multiple scattering
and the absorption and scattering due to gases and particles. TUV inputs of
interest were surface reflectivity (albedo), O3 column, aerosol
optical depth at 500 nm (AOD), single scattering albedo of aerosol (SSA),
NO2 column and air density. Total O3 column values were
taken from the Ozone Monitoring Instrument (OMI) on the Aura spacecraft of
NASA (Levelt et al., 2006). Aerosol columnar optical properties were obtained
from the Aerosol Robotic Network (AERONET). AOD data were measured at the
FORTH_Crete station which is located 35 km west of Finokalia (Fotiadi et
al., 2006). Data level 1.5 from Version 2 were used (cloud-screened). Total
NO2 column values were taken from the Global Ozone Monitoring Experiment-2 (GOME-2) and OMI instruments. The
calculations were carried out at wavelength from 280 to 650 nm with a
resolution of 5 nm. Simulations using surface reflectivity of 0.075 and
simulations using O3 column taken from OMI had the best correlation
with measurements. However, the TUV model still significantly overestimated
JO1D and JNO2 data. Thus, a
parameterization took place following a simple empirical approach, according
to Mogensen et al. (2015), and the ratios between the measured and modeled
(from TUV) photolysis rate were calculated and used in the model.

3.1 Particle size distribution and its connection with NPF

We analyzed all available measurements of number size distributions of
atmospheric aerosol particles measured at Finokalia in order to identify and
analyze the NPF phenomenon in the eastern Mediterranean. The data coverage
for the period 2008–2018 was 82 %, providing one of the longest time
series of size distributions not only in this region but also in southern Europe and a unique database for aerosol physical properties.

Figure 1Monthly average variation in (a) nucleation mode particle
number concentration and (b) sulfuric acid condensational sink
(CS) at Finokalia station over the period June 2008–June 2018. Whiskers
represent 10th and 90th percentiles, box edges are 75th and 25th percentiles,
the line in the box is the median, and the solid square is the mean.

First, we calculated the total particle number concentration (median
concentration was 2202 cm−3, standard deviation (SD) 528 cm−3)
and corresponding number concentration in the nucleation mode
(Dp<25 nm, median 80 cm−3, SD 528 cm−3),
Aitken mode (25 nm <Dp<100 nm, median 1028 cm−3, SD 894 cm−3) and accumulation mode (Dp>100 nm, median 898 cm−3, SD 605 cm−3).
We found that Aitken mode accounted for 50 % and accumulation mode for 42 %
of the total particle number concentration, while the nucleation mode
accounted only for 8 %. The standard deviation of the nucleation particle
number concentration was 528 cm−3, indicating that the abundance of
these smallest particles is of an episodic nature. The highest monthly average
concentrations of nucleation mode particles were observed during winter and
early spring and the lowest ones during summer (Fig. 1a). Calculating the
median diurnal variability of the nucleation mode, we can see that there is
a clear pattern for all seasons of the year (Fig. 2a) with a sudden burst in
the number concentration around noon that is most pronounced in winter and
least in summer. Such an observation suggests that the nucleation particle
number concentration is controlled by NPF episodes rather than other sources
such as combustion processes. As can be seen in Fig. 2b, where a typical
“banana-shaped” pattern of an NPF event at Finokalia is presented, the
sudden burst at noon is typical for a NPF event. In summer, nucleation mode
particles have the highest concentrations during the night, yet another
concentration relative maximum at noon can be attributed to NPF (Fig. 2a).
The shift in the average time of the daytime burst of nucleation mode
particles can be attributed to the annual variation in the daylight length.
Similar observations to ours have been reported in Cusack et al. (2013) for
the western Mediterranean where the diurnal variation in nucleation mode
particles presents a clear maximum at noon under both polluted and clean
conditions.

Figure 2(a) Average diurnal variation in nucleation mode particle
number concentration (hourly values) at Finokalia over the period June
2008–June 2018. (b) New particle formation event captured at
Finokalia on 29 August 2012 (time in UTC+2).

It is worth noticing that during nighttime the median nucleation mode
particle number concentrations were similar in all the seasons. This
suggests that there is some new particle production mechanism at night,
especially in summer and autumn, that operates separately from daytime NPF.
Frequently during the nighttime, we observed a pronounced appearance of new
nucleation mode particles over several hours as illustrated by Fig. 3. While
nocturnal NPF has been reported in the literature (see Salimi et al., 2017, and references therein), this phenomenon seems to be rare and it remains
unclear what the exact mechanisms leading to it are. Given that we observed
no or little growth during nighttime NPF, we may assume that the sources
leading to the formation of new particles are local rather than regional and
that the lack of photochemistry during night limits the abundance of
condensable vapors driving particle growth. Observations of very localized
NPF have been reported in Mace Head, Ireland, where intense NPF frequently
takes place under low tide conditions when algae are exposed to the
atmosphere (O'Dowd et al., 2002). Henceforth, we will exclude the nighttime
NPF events from our further analysis. We refer the interested reader to
Kalivitis et al. (2012) for a more detailed description of this phenomenon.

Figure 3Example of appearance of nucleation mode particles during several
hours as observed during the night of 10 to 11 March 2009 (time in UTC+2).

Overall, we observed atmospheric NPF to take place during both day and night
at Finokalia but no sign of any other source of nucleation mode particles
in measured air masses. We therefore hypothesize that atmospheric NPF is the
dominant source of nucleation mode particles in this Mediterranean
environment.

3.2 Characteristics of NPF in the eastern Mediterranean

We analyzed the data set of aerosol size distributions following the approach
of Dal Maso et al. (2005) in order to mark the available days as (1) NPF
event days when a clear new nucleation mode and subsequent growth of
newly formed particles to larger diameters can be observed, (2) non-event
days, and (3) undefined days when either new particles appear into the Aitken
mode or nucleation mode particles do not show a clear growth. The available
days were manually inspected and classified.

We used the Statistica software package for Windows to carry out factor
analyses, including meteorological parameters, ozone concentrations (as an
important oxidant in the atmosphere) and PM10 mass concentration (as an
index of particulate pollutant levels), in order to examine whether any of
these factors were associated with the formation of new particles,
represented by the nucleation mode number concentration. Furthermore, we
divided our data into night and day time periods in order to separate daytime
NPF from that taking place during nighttime. The only parameter that had
some effect on the nucleation mode particle number concentration was the
wind velocity: when strong winds were prevailing at Finokalia, it was more
unlikely to observe nucleation particles. On the other hand, the lack of
correlation with any other parameter may indicate that the NPF is not
sensitive to local meteorological conditions, preexisting particulate matter
and ozone levels in this environment. Air mass back trajectories calculated
using the HYSPLIT model (Stein et al., 2015) showed little difference during NPF events from air
masses typical for the prevailing situation at Finokalia: air masses
arriving at Finokalia from the northeast were the most frequent during NPF
events (30 % against 24 % of all days), followed by northern directions
(20 % against 21 %) and northwestern air masses that were more frequent
than the average (19 % against 17 %).

Table 1Total available measurement days and percentage of NPF events
observed at Finokalia during the period June 2008–June 2018.

Next, we focused on determining the main characteristics of daytime NPF at
Finokalia. Overall, 837 NPF events were identified. This is one of the
longest time series of the NPF phenomenon recorded in the Mediterranean
atmosphere, providing a representative climatology of NPF events in this
region. NPF took place on 27 % of the 3057 available measurement days whereas
no event occurred on 50 % of those days. It is worth noting that 23 % of
the days were characterized as undefined, which means that while no clear
NPF event could be observed, there was some evidence of secondary particle
formation although not in the immediate vicinity of the station (Table 1).
We found that NPF is most frequent in April and May, probably due to the
biogenic activity and the onset of intense photochemistry, and least
frequent in August (Fig. 4) probably due to high wind speeds occurring in these
months (not shown) and additionally the high condensational sink (Fig. 1b).
The rain season in southeastern Europe in early autumn leads to a gradual CS
decrease, and as a result a local maximum in NPF frequency is observed in
October. NPF at Finokalia takes place throughout the year.

Figure 4Seasonal variation in NPF percentage of occurrence of event,
non-event and undefined days relatively to available measurement days at
Finokalia for the period June 2008–June 2018. Whiskers represent 10th and
90th percentiles, box edges are 75th and 25th percentiles, the horizontal
line in the box is the median, and the square is the mean.

As a next step, we classified the NPF events as Class I or Class II events
depending on whether the particle formation rate at 9 nm (J9) and
growth rates from 9 to 25 nm diameter (GR9–25) could be
calculated with good confidence or not. Overall, Class I
events corresponded to 8 % of the available measuring days and 28 %
of the event days, and they were observed throughout the year, providing
enough data for a statistical analysis of particle formation and growth rates
during NPF events (Fig. 5).

Figure 5Seasonal variation in percentage of occurrence of NPF Class I and II
events relatively to available measurement days at Finokalia in the eastern
Mediterranean for the period June 2008–June 2018.

Figure 6Seasonal variation in (a) formation rate of 9 nm particles
(J9) and (b) growth rate in the size range 9–25 nm
(GR9–25) as calculated during Class I NPF events at Finokalia
for the period June 2008–June 2018. Whiskers represent 10th and 90th
percentiles, box edges are 75th and 25th percentiles, the horizontal line in
the box is the median, and the solid square is the mean.

The average value of J9 during the Class I NPF events in Finokalia was
0.9 cm−3 s−1 (median 0.5 cm−3 s−1, SD
1.2 cm−3 s−1). This is well within the range of values reported for
J10 at other locations (Kulmala et al., 2004a), although higher than
J16 reported by Berland et al. (2017) at the Finokalia site in 2013
(0.26 cm−3 s−1), but substantially lower than the values found by
Kopanakis et al. (2013) in western Crete (13.1±9.9 cm−3 s−1). The monthly variation in J9 (Fig. 6a) shows
that the highest average formation rates were observed in December and
January, probably as a result of the low CS values observed in winter,
although it is difficult to say which factors determine the monthly
variability of J9 at Finokalia. Seasonal averages of J9,
GR9–25 and CS are summarized in Table 2. Moreover, we found
that J9 and N9–25 have a clear linear relation (Fig. 7),
which supports our earlier hypothesis that at Finokalia the main source of
nucleation mode particles is their secondary formation in the atmosphere.

Table 2Formation rates for 9 nm particles (J9), growth rates in the
size range 9–25 nm (GR9–25) for NPF events observed at
Finokalia and condensational sink for sulfuric acid (CS) on a seasonal
basis during the period June 2008–June 2018 (mean, median and standard
deviation).

Figure 7Scatterplot of the number concentration of nucleation mode
particles (N9–25) (hourly maximum value during the event) versus
formation rates of 9 nm particles (J9) at Finokalia, for events when
J9 could be calculated with a good level of confidence (Class I
events).

We calculated the average growth rate of the newly formed particles to be
5.4 nm h−1 (median 4.5 nm h−1, SD 3.9 nm h−1). We found
that GR9–25 is highest in summer until September and lowest in
winter and early spring, probably in line with the seasonal cycle of
photochemical activity and biogenic emission patterns, producing condensable
species that are driving the growth process (Fig. 6b). Additionally,
transported pollution in summer at Finokalia may contribute except for that of CS to
GR, since transported anthropogenic SO2 is a precursor
for condensable sulfuric acid.

The survival probability of newly formed particles is closely related to the
ratio of CS to GR, at least for cluster sizes (Kerminen and Kulmala,
2002; Kulmala et al., 2017), and at Finokalia they present the same annual
cycle. The survival probability for nucleation mode particles for Class I
events was calculated based on the formula in Kulmala et al. (2017). It was
found that on a seasonal basis the median survival probability is higher in
summer and winter but varies between the seasons only within 5 %.
The concentrations of nucleation mode particles are lower during summer and
the average duration of the NPF in summer seems to be shorter as shown in
Figs. 1 and 2a, respectively. These observations may be explained by the
higher CS and GR during summer. The CS (and hence CoagS) may directly
affect the maximum concentrations observed. The slightly higher survival
probability in summer perhaps explains that given high CS values, new
particles need to grow fast in order to survive. On the other hand, one would
expect NPF to be most frequent in winter when the highest concentrations of
nucleation particles are observed and CS is the lowest, but this was
not the case. A possible explanation for the high nucleation mode particle
number concentrations in winter could be that the survival probability is
higher than in spring or autumn.

Figure 8(a) Time series of monthly NPF percentage of occurrence at
Finokalia for the years 2008–2018. (b) Annual NPF percentage of
occurrence at Finokalia for the period June 2008–June 2018 for Class I and
II events. Interannual variation in (c) formation rates of 9 nm
particles (J9) and (d) growth rate in the size range 9–25 nm
(GR9–25) during Class I NPF events at Finokalia for the period
June 2008–June 2015 (the solid circles represent annual averages and the
dashed lines the linear regression).

3.3 NPF trends during the 2008–2018 period

During the period under study no statistically significant trends in NPF
events were observed at Finokalia for the 120 available months. It should be
noted though, that since 2010 a decreasing trend is observed, which is
statistically significant with a p value of 0.005. During the measurement
period under study, no trend in J9 was observed (Fig. 8c). Although no
statistically significant trend was observed for GR9–25 as well (Fig. 8d), we observed a decreasing trend during the period 2008–2015 of
about 0.3 nm h−1 yr−1. This trend can be considered statistically
significant (p value of 0.03). In order to explain this trend, we need to
emphasize the regional characteristics of the observations at Finokalia, as
this site is greatly affected by long-range transported pollutants of
marine, desert dust and polluted continental origin (Lelieveld et al.,
2002). Non-sea salt sulfate (nss-SO42-) can be considered as an
indicator of regional pollution from anthropogenic activities (SO2 emissions), and since the beginning of the economic crisis in Europe,
especially in Greece, we observed a clear decline in its concentration since
2008 (Paraskevopoulou et al., 2015), which, however, stopped after 2015. We
can therefore also assume a regional decrease in SO2 emissions, since
the main source of SO2 at Finokalia is attributed to transported
pollution (Sciare et al., 2003). This could result in a decrease in the
availability of sulfuric acid, a major condensable species responsible for particle growth (Bzdek et al., 2012).

Hamed et al. (2010) studied the effect of the reduction in anthropogenic
SO2 emissions in Germany between the years 1996–1997 and 2003–2006 as a
result of the socioeconomic changes in East Germany after reunification. They observed a notable decrease in the NPF event frequency
but an increase in the growth rate of nucleated particles. A decrease in the
NPF frequency due to the reduction in anthropogenic SO2 emissions
in eastern Lapland was also reported by Kyrö et al. (2014), and
this decrease was most pronounced for the Class I NPF events. Nieminen et
al. (2014) analyzed the longest data set reported in literature from Finland
and found that, despite major decreases in ambient SO2 concentrations
observed all over Europe as a result of overall air quality improvements,
there was a slight upward trend in the particle formation and growth rates.
This feature was attributed partly to increased biogenic emissions over the
same period.

In our case the reasons for the variations in the NPF frequency, J9 and
GR9–25 remain unclear, even though factors like meteorological conditions
and organic vapor abundance have probably played some role in this respect.

Figure 9Nucleation event observed at Finokalia on 26 March 2013 as captured
by AIS (left panels for negative (a) and positive (b)
polarity) and SMPS (c) (time in UTC+2).

3.4 Atmospheric ion observations related to new particle formation

At the Finokalia station, atmospheric ion observations relevant to new
particle formation were performed during two separate periods: 2008–2009
during the EUCAARI project (Manninen et al., 2010) and 2012–2014 during the
FRONT (Formation and growth of atmospheric nanoparticles) project. Here we
will focus only on FRONT data, since the EUCAARI data set is discussed in
detail in Manninen et al. (2010) and Pikridas et al. (2012). A typical
nucleation event is presented in Fig. 9 as recorded by both the AIS and
SMPS. AIS observations may provide information about the initial stages of
new particle formation as particles can be observed emerging in the
intermediate ion diameter range 1.6–7.4 nm. Intermediate ions appear only
under certain circumstances, such as during precipitation, at high wind
speeds and when NPF is taking place (Hõrrak et al., 1998; Tammet et al.,
2014; Leino et al., 2016; Chen et al., 2017). In the following we will focus
on NPF and use only the observations from the negative polarity due to the
better representation of NPF events in those data compared with
corresponding positive ions in our data set (Kalivitis et al., 2012).

We classified all of the available AIS measurement days into event,
non-event and undefined days, once again according to methods introduced by
Dal Maso et al. (2005), and subsequently compared the findings from AIS data
to those from the SMPS. In Fig. 9 an NPF event is presented observed with
both the AIS and the SMPS at Finokalia. Surprisingly, the two data sets for
the same time period gave quite different results in terms of the NPF event
frequency: in the AIS data the NPF event frequency peaked earlier during the
year than in the SMPS data (Fig. 10). This feature was evident in both
periods of AIS measurements and has been also reported at a rural site in
Hungary (Yli-Juuti et al., 2009), probably because AIS detects only
naturally charged particles while SMPS detects all particles. Additionally,
it is possible that AIS data are more representative of local NPF events
with limited particle growth, and such events may not be seen in the SMPS
data. On the other hand, the SMPS measures neutral particles but has a much
higher detection limit (9 nm), so its data may be more representative of
regional NPF that takes place over distances of hundreds of kilometers
(Kalkavouras et al., 2017).

Figure 10Monthly variability of NPF events' percentage of occurrence
relatively to available measurement days at Finokalia as determined by
analysis of AIS data during the FRONT experiment (November 2012–July 2014).
For a direct comparison, the monthly variability of NPF events as obtained
from the SMPS measurements for the same period is included. Above the
columns, the number of NPF events observed for AIS (top), SMPS (middle) and
the number of common events for both instruments (bottom, italic, bold) for
each month are presented.

We calculated the growth rates at three different size ranges for the FRONT
project similarly to Manninen et al. (2010) and Pikridas et al. (2012) for
the EUCAARI project data. The particle growth rates in the size ranges 1.5–3, 3–7 and 7–20 nm were 1.6±1.8, 5.4±4.9 and 9.1±9.5 nm h−1, respectively. These
values are lower than those in Pikridas et al. (2012) but comparable to
those observed during the EUCAARI project for the first two size ranges and
higher than those observed during the EUCAARI project for the last size
range (Manninen et al., 2010). Overall, we observed much faster growth of
newly formed charged particles in the eastern Mediterranean atmosphere after
their first growth steps beyond 3 nm in diameter, reflecting probably the
strong Kelvin effect at small particle sizes preventing condensation and
hence growth, and the abundance of precursors leading to nucleation and
condensing species contributing to each growth stage.

3.5 Simulations of NPF using the zero-dimensional model MALTE-Box

In order to evaluate our understanding of the observed NPF events in the
eastern Mediterranean we chose to simulate two distinct cases of 1 week's duration each, during which NPF events were observed (event week) or
not (non-event week). The selection was done from the summer of the year
2012, when JO1D and JNO2 photodissociation measurements were also
available at Finokalia. Two weeks in August and September 2012 were chosen:
28 August–3 September as an event week and 9–15 August as a non-event week. The “event week” was
described in detail by Kalivitis et al. (2015). Applying the MALTE-Box model,
the aerosol size distribution and its evolution over the week was simulated for these two cases.

Figure 11Simulations with the MALTE box with the adjusted parameters for the
subtropical environment for the “event week” that NPF events were observed
at Finokalia. Measured and modeled (a) total number concentration
and (b) total volume concentration for the same period. The x axis
in both figures gives the Julian day of 2012.

During the event week the simulated formation of new particles
successfully coincided with the observations. The NPF events simulated using
the nucleation rates as parameterized for the boreal environment
overestimated the observations, while the simulated growth of newly formed
particles was greatly underestimated as shown in Tzitzikalaki et al. (2017).
The most likely reason for this is the very low concentration of
monoterpenes, calculated by the TM4-ECPL global model for the Finokalia model
grid box, on which the ELVOC and SVOC chemistry was built. Indeed, the
TM4-ECPL model results for Finokalia were too low compared to monoterpene observations in 2014 (not shown). Therefore, we performed a number of
sensitivity tests to improve the simulations by adjusting the nucleation
coefficient and the monoterpene concentrations until we simulated
efficiently the nucleation and growth rates observed during the second day
of the event week when the most pronounced NPF event was observed. The
best agreement between model results and observations was reached by
decreasing the nucleation coefficient from 10−11 s−1 (the
value commonly used for the boreal environment) to 5×10-16 s−1 and increasing the α- and β-pinene concentrations by a factor of 10. With these modifications the model results improved
and the aerosol number size distributions were better simulated, as were total number and volume concentration of aerosol particles (Fig. 11a and b, respectively). This was the first time that we were able to simulate NPF in
the eastern Mediterranean environment. The almost 5 orders of magnitude
lower nucleation coefficient used here for the subtropical setup could be
related to the contribution of still unknown compounds in the
cluster-formation process. Huang et al. (2016) applied different kinetic
nucleation coefficients at Nanjing, China, with the lowest value for a
“China-clean” day of 6.0×10-13 s−1. The higher
monoterpene concentrations used are comparable to the findings at Finokalia
but also at another location in the eastern Mediterranean (Debevec et al.,
2018).

Figure 12Simulations with the MALTE box with the adjusted parameters for the
subtropical environment for the “non-event” week that no NPF was observed
at Finokalia. Measured and modeled (a) total number concentration
and (b) total volume concentration for the same period. The x axis in both figures gives the Julian day of 2012.

Using the non-event week as our control case, we performed simulations of
number size distributions at Finokalia station using the subtropical setup
and compared it to our measurements. For the “non-event week”, weak NPF
were predicted by the model during the last 2 days that were not found in
the measurements (Tzitzikalaki et al., 2017) but appear to be associated with
the rapid drop in CS during day 5 of the simulations. Nevertheless, even
if no NPF took place during the last 2 days, it was apparent in our
measurements that some nucleation particles appeared (∼200 cm−3), and thus the general tendency was captured by the model. Both total number
and volume concentrations were adequately simulated by the model (Fig. 12a,
b). These results show the potential of the MALTE-Box model to simulate the NPF
in the eastern Mediterranean and the importance of input data. Therefore,
when more appropriate input data for Malte-box become available
(concurrent detailed measurements of gases and aerosol distributions) at
Finokalia, new simulations and VOC measurements will provide further insight
into the nucleation mechanisms, the growth process and the factors controlling
NPF in the eastern Mediterranean atmosphere.

NPF in the atmosphere is a recurrent phenomenon in the eastern Mediterranean. In
this study, we presented the longest time series of NPF records in the
region. We analyzed 3057 days of aerosol number size distribution data from
June 2008 to June 2018 and found that NPF took place on 27 % of the
available days, more frequently in spring and less frequently in late
summer. The production of nucleation mode particles was common during nighttime
as well. Nucleation mode particle number concentrations were low outside
periods of active NPF and subsequent particle growth, indicating an absence of
local sources. The classification of NPF events based on atmospheric ion
measurements differed from the corresponding classification based on
mobility spectrometer measurements: the maximum frequency of NPF events was
observed earlier in spring from AIS data than from SMPS data, possibly
indicating a different representation of local and regional NPF events
between these two data sets since SMPS measures new particles after they
have grown to diameters larger than 9 nm and hence records only regional
events lasting for several hours.

We used the MALTE-Box model to simulate NPF observations in the eastern
Mediterranean region. Using a “subtropical” environment parameterization,
we were able to simulate with good agreement the selected time period. The
parameterization used was significantly different than the one used for the
boreal environment: nucleation rates were much lower, yet monoterpenes
seemed to play a key role in the mechanisms governing NPF phenomena.

From the results presented in this work it is evident that the Finokalia
site is a unique location in the eastern Mediterranean for studying the
processes leading to NPF in the marine environment. As a next step, a more
detailed look at the precursors driving these processes is necessary, with
special emphasis on VOCs and the expansion of the available measurements at
the site in order to eliminate the uncertainties introduced in our
simulations by the use of model outputs instead of observations.

The data are available from the corresponding authors upon
request. The authors acknowledge the free use of O3 data from the OMI
sensor accessed via the NASA Aura Validation Data Center at
http://avdc.gsfc.nasa.gov (last access: 20 February 2019). Total
NO2 column data are the property of the University of Bremen
(Andreas Hilboll, personal communication, 2014). The
authors gratefully acknowledge the NOAA Air Resources Laboratory (ARL) for
the provision of the HYSPLIT transport and dispersion model and the READY website
(http://www.ready.noaa.gov, last access: 20 February 2019) used
in this publication.

The research project was implemented within the framework of the action
“Supporting Postdoctoral Researchers” of the operational program
“Education and Lifelong Learning” (action's beneficiary: General Secretariat
for Research and Technology) and was cofinanced by the European Social
Fund (ESF) and the Greek State. This research is supported by the Academy of
Finland Center of Excellence program (project number 1118615). We
acknowledge funding from the EU FP7-ENV-2013 program “Impact of Biogenic
vs. Anthropogenic emissions on Clouds and Climate: towards a Holistic
UnderStanding” (BACCHUS), project no. 603445, and the Horizon 2020 research
and innovation programme ACTRIS-2 Integrating Activities (grant agreement
no. 654109). This study contributes to ChArMEx work package 1 on aerosol
sources. We acknowledge support of this work by the project “PANhellenic
infrastructure for Atmospheric Composition and climatE change” (MIS
5021516), which is implemented under the action “Reinforcement of the
Research and Innovation Infrastructure”, funded by the operational programme “Competitiveness, Entrepreneurship and Innovation” (NSRF
2014–2020) and cofinanced by Greece and the European Union (European
Regional Development Fund).

New particle formation (NPF) is an important source of atmospheric aerosols. For the Mediterranean atmosphere, only few studies exist. In this study we present one of the longest series of NPF by analyzing 10 years of data from Crete, Greece. NPF took place on 27 % of the available days; it was more frequent in spring and less so in late summer. Model simulations showed that NPF in the subtropical environment may differ greatly from that in the boreal environment.

New particle formation (NPF) is an important source of atmospheric aerosols. For the...